(i) Field of the Invention
The present invention relates to carbon fibers for reinforcement of cement and a cement composite material using the carbon fibers. More specifically, the present invention relates to carbon fibers for reinforcement of cement which have excellent adhesive properties to the cement, good process passage properties through a direct spray gun and good bundling properties and which are particularly suitable for a direct spray method, and a cement composite material using the above-mentioned carbon fibers and having high bending strength.
(ii) Description of the Related Arts
In recent years, carbon fibers have been used not only as materials in various fields of, for example, airplane parts, automobile parts, sporting goods and the like, but also as reinforcement materials for resins and cements, because of having excellent features such as high strength, high modulus of elasticity and light weight. Thus, the demand of the carbon fibers has been remarkably increased.
Furthermore, kneaded materials of hydraulic cement powders have been widely used as various building materials and civil engineering materials. In order to reinforce such a kneaded material and to prevent the occurrence of cracks, it has been heretofore attempted to blend a fibrous material with the kneaded material. However, the employment of asbestos is not preferable, because the asbestos is a carcinogen. In addition, glass fibers are poor in alkali resistance, so that the strength of the glass fibers themselves inconveniently deteriorates in the cement. Thus, various kinds of organic fibers and alkali-resistant glass fibers are instead used as the reinforcement materials. However, the organic fibers are poor in fire resistance, and even in the case of the alkali-resistant glass fibers, the strength of the kneaded material declines, when these fibers are used for a long period of time under alkaline circumstances of the cement or the like. Hence, much attention is now paid to carbon fibers having excellent heat resistance and excellent chemical resistance as well as high strength and high modulus of elasticity.
However, when added to and mixed with the cement, the carbon fibers are poorer in adhesive properties (or stickiness) to the cement as compared with the asbestos and the glass fibers, and therefore there is the problem that they cannot exert a sufficient effect as the reinforcement material. Accordingly, various contrivances have been made in order to increase the adhesive properties of the carbon fibers to the cement and to thereby heighten the strength of the resultant carbon fibers-reinforced cement material. For example, there have been suggested a method in which carbon fiber strands impregnated with a hydrophobic liquid resin such as an epoxy resin are stretched in cement, and the resin and the cement are then simultaneously hardened (Japanese Patent Publication No. 19620/1983), a method in which carbon fibers mutually bound by a water-soluble binder such as methyl cellulose are arranged in one direction or two crossable directions in cement (Japanese Patent Application Laid-open No. 129657/1981), a method in which cement slurry layers are superposed upon each other with the interposition of a carbon fiber sheet including a water-soluble synthetic resin emulsion such as an acrylic emulsion (Japanese Patent Application Laid-open No. 223659/1983), a method in which in manufacturing a reinforced cement material by a hacek manufacturing method, a nonionic or a cationic high polymeric coagulant such as a polyalkylamino acrylate is applied onto the surfaces of fibers (Japanese Patent Application Laid-open No. 81052/1985), and a fibrous material for cement reinforcement in which a cationic rubber latex is applied onto the surfaces of carbon fibers (Japanese Patent Application Laid-open No. 108755/1987).
In these conventional techniques, however, restriction is put on the morphology of the carbon fibers to be used and a construction method to be selected, and the kind of cement to be used is limited. In addition, they have the problem that the adhesive properties to cement are not sufficient. For these reasons, the conventional techniques are not always sufficiently satisfactory.
In recent years, in the manufacture of a concrete composite material containing a certain kind of fibers as a reinforcement material, a direct spray method in which longer fibers (25 to 30 mm or more) can be used and which can sufficiently exert dynamic characteristics has been noticed and put to practical use by the utilization of glass fibers. Furthermore, it has also been suggested that pitch-based carbon fibers, in which bundling properties are enhanced by the use of a special bundling agent in a fiber manufacturing process, are applied to the direct spray method (Japanese Patent Application Laid-open No. 30008/1987), but since the manufacturing process and the morphology of the carbon fibers are limited, this suggested technique cannot be applied to the generic field of the carbon fibers. In the case that this direct spray method is applied, it is important that the carbon fibers for reinforcement are excellent in adhesive properties to cement, and that they have good process passage properties through a direct spray gun, i.e., friction between the carbon fibers and metals is so low as to make the carbon fibers easily slidable.